1. Field of the Invention
The present invention relates in general to regulated direct current (DC) power supplies; and more particularly, to redundant DC power supplies for a MULTI-BUS digital system with relatively large current requirements.
2. Description of the Related Art
Many applications of electrical control systems require a high degree of reliability. For example, whenever an electrical control system is operating in a hostile environment, such as in a nuclear power system or in outer space, the reliability of the control system is of paramount importance. One method of increasing reliability is to provide some type of redundancy. Even in less critical applications, redundant components are often used to provide uninterrupted service.
One of the weakest components in many control systems which utilize solid-state devices, is the power supply for the system. There have been a variety of attempts to increase the reliability of power supplies by utilizing redundant components. However, no known design is capable of providing a well regulated voltage, e.g., .+-.0.1% change in voltage for a 10% change in a relatively large current, e.g., 30 or 40 amperes, without utilizing a complex control circuit which reduces the inherent reliability of the power supply system having redundant power supplies. For example, when relatively large currents are drawn from a power supply system, by a control system having relatively long wiring lengths, e.g., in an industrial plant process control system or a nuclear power generation system, remote sensing of the voltage as close as possible to the load makes it possible to provide better voltage regulation than when local sensing of the voltage output by the power supply system is used. However, most known redundant power supply systems do not allow for remote sensing.
Examples of prior art attempts to improve reliability include U.S. Pat. No. 4,075,502 to Walley, Jr. which utilizes a complex control circuit to select one of two DC power supplies as providing a voltage which is closer to the preset standards. Thus, the redundant power supply system taught by Walley, Jr. adds an additional unit, the control circuit, which can fail; thus reducing the inherent reliability gained by utilizing redundant power supplies. Similarly, a power supply system taught by U.S. Pat. No. 4,096,394 to Ullmann et al. utilizes active elements to provide switching from one power supply to another. The system taught by Ullmann et al. also uses an energy store which maintains the voltage supplied to the load during switching from one power supply to the other; thus, improving the ability of the power supply system to provide continuous operation at close to the desired voltage level. However, the system taught by Ullmann et al. is directed to a diesel generator backup for an alternating current (AC) system, not a low voltage, high current direct current (DC) system.
Another redundant power supply system which utilizes active switching equivalents is disclosed in U.S. Pat. No. 3,912,940 to Vince which eliminates the need for the additional energy store while still providing continuous operation. The power supply system disclosed in Vince is capable of maintaining the voltage at close to a preset level when the active power supply fails by keeping the stand-by power supply on at all times with a reduced output. When the active power supply fails, the stand-by power supply is turned fully on. In addition to the reliance on active switching elements, i.e., transistors, the power supply system in Vince does not address the specific requirements of a low voltage, high current system.
Other power supply systems, similar to that taught by Vince, are available from Powertec, Inc. The Powertec PS4001 is fully load-sharing, that is, there is no "stand-by" power supply, rather both power supplies provide power simultaneously. However, the PS4001 power supply is not remote sensing and as a result the voltage is not regulated as well as possible.
The Powertec 29D series adds remote sense capability and "unlimited" expansion to the features of the PS4001. However, complex circuitry is required to provide the additional features with a resulting increased possibility of failure, as discussed above. In addition the 29D series power supplies are not easy to install separately for maintenance concurrent with operation of the MULTI-BUS system.
A power supply system which does not require the operation of active switching elements to maintain the voltage level during switching from one power supply to another is disclosed in U.S. Pat. No. 3,641,570 to Thompson. The system taught by Thompson utilizes a filter capacitor which minimizes fluctuations on the positive power bus. However, the power supply system disclosed in Thompson does not utilize remote sensing or provide a regulated voltage by any other means.